Brussels mayor Philip Close said the alert status, already just one off the maximum level, had not been increased.

“Initial indications are ... that it is an isolated attack, a single person,” Close told reporters beside a street blocked by police.

In addition to regular police, Brussels streets are being patrolled by soldiers due to a heightened security threat level in the wake of militant attacks in Paris and the Belgian capital in 2015 and 2016.

In June, troops shot dead a suspected suicide bomber in Brussels’ central train station but there were no other casualties, in what authorities treated as a terrorist incident.